Method for treating a nuclear graphite contaminated

ABSTRACT

The invention relates to a process for the treatment of a nuclear graphite contaminated with radioelements, the said process comprising a milling step consisting in subjecting the said graphite, immersed in a liquid medium, to high-voltage pulses, the said liquid medium having a resistivity such that, owing to the effect of the energy conveyed by the said pulses, electric arcs are initiated and, upon contact with the said graphite, break the carbon-carbon bonds that make up this graphite, the number of high-voltage pulses being set so as to obtain a given particle size.

TECHNICAL FIELD

The subject of the present invention is a process for the treatment of anuclear graphite contaminated with radioelements by milling the saidgraphite immersed in a liquid medium, especially of the graphite comingfrom the NUGG (Natural Uranium-Graphite-Gas) system recovered duringdismantlement or the nuclear graphite coming from nuclear sites duringnuclear clean-up operations.

PRIOR ART

The general field of the invention is therefore that of the treatment ofnuclear waste, such as nuclear graphite contaminated with radioelements.

At the present time, one of the nuclear graphite treatment processesconsists in subjecting the said graphite to dry fragmentation, in air,by the use of conventional milling means, such as percussion mills orroll mills, so as to obtain a powder which is then subjected to acombustion operation in order to completely destroy the contaminatedgraphite.

However, this treatment process has the following drawbacks:

-   -   this process is a very expensive process, insofar as the        graphite has a hardness such that it causes rapid wear of the        mechanical components used in making the mills, thereby        requiring the said components to be frequently renewed;    -   this process causes the formation of very fine graphite        particles which, when they are in suspension in the air, may        cause an explosion when a spark occurs; and    -   this process causes substantial pollution due in particular to        the volatility of the graphite particles, it being possible for        these particles to be laden optionally with radioactive heavy        metals, such as ⁶⁰Co and ¹³⁷Cs, thereby requiring containment of        the milling station so as to avoid any leakage into the open air        of contaminating radioactive elements; however, the        establishment of such containment does not thereby prevent        dispersion of volatile radioelements, such as tritium, which may        escape via the ventilation systems of the station.

DESCRIPTION OF THE INVENTION

Thus, the object of the present invention is to propose a process forthe treatment of a nuclear graphite contaminated with radioelements,which does not have the drawbacks of the prior art and which, inparticular, does not require the use of mechanical components and whichdoes not cause the dispersion of radioelements and also obviates therisks of a powder explosion.

To achieve this, the subject of the invention is a process for thetreatment of a nuclear graphite contaminated with radioelements, thesaid process comprising a milling step consisting in subjecting the saidgraphite, immersed in a liquid medium, to high-voltage pulses, the saidliquid medium having a resistivity such that, owing to the effect of theenergy conveyed by the said pulses, electric arcs are initiated and,upon contact with the said graphite, break the carbon-carbon bonds thatmake up this graphite, the number of high-voltage pulses being set so asto obtain graphite particles of a given particle size.

It should be mentioned that, according to the invention, the term“high-voltage pulses” is understood to mean electrical pulses that canconvey a voltage of the order of one or more kilovolts resulting in thecreation of electrical arcs in a liquid medium having resistivityproperties suitable for arc formation. Thus, for the purpose ofimplementing this process, liquid media with a resistivity of greaterthan 1 MΩ.cm may advantageously be used.

This process has the advantage of being able to carried out without theuse of mechanical milling components, thereby minimizing the runningcosts of this process compared with those of the prior art.

In addition, this treatment process has the advantage of being carriedout in a liquid medium. Consequently, the graphite powders resultingfrom the milling are trapped in this liquid medium, and this preventsthe abovementioned powder explosion phenomenon. In addition, theradioelements released during the milling of the graphite remainconfined in the liquid medium, for example by isotope exchange, as isthe case with tritium.

Apart from the release and the trapping of the radioelements, theprocess according to the invention makes it possible to obtain, afterits completion, graphite particles of a given particle size, whichparticles may either be subjected to a combustion operation, so as tocompletely destroy them, or may be recovered, for the purpose ofpossibly reusing them, for example as a base product for geologicalbarriers constructed for a long-term storage of highly radioactivesubstances. These particles may also be stored under conditions in whichthere is no bleaching by surface water.

According to the invention, to mill the nuclear graphite into the formof relatively fine particles, a person skilled in the art can readilychoose the high-voltage pulse application conditions (energy, frequency,duration and number of pulses delivered) depending on the nature of theinitial graphite, it being understood that the higher the energy of thepulses the fewer the number of pulses to be applied in order to obtain agiven particle size.

According to the invention, the energy conveyed by each pulse mayadvantageously be between 10 J and 100 kJ, preferably equal to 1 kJ.

According to the invention, the high-voltage pulses may advantageouslyhave a duration ranging from around from 100 ns to 100 μs, preferablywith a duration of 1 μs.

According to the invention, the high-voltage pulses may have a frequencyranging from 1 Hz to 1000 Hz, preferably 10 Hz. It should be clearlyunderstood that this frequency will be specifically set by a personskilled in the art depending on the generator used.

According to one particularly advantageous embodiment of the invention,one liquid medium that can be used within the context of this process iswater. It should be clearly understood that the water used within thecontext of the invention will advantageously have resistivity propertiessuch that an electric arc can be initiated through the effect of thehigh-voltage pulses. For example, the water used may be partiallydeionized, so as to have a lower conductivity than water that has notundergone any treatment.

Advantageously, the process of the invention may also include atreatment step carried out on the liquid medium in which the graphitemilling takes place, this treatment being a conventional treatmentintended, especially when this liquid medium is water, for purifying theliquid medium of the radioelements released and for maintaining itsresistivity, these treatments being within the competence of a personskilled in the art. The treatment of the liquid medium, intended topurify the said medium of the radioelements contained therein, may bethat ordinarily practiced in LETPs (Liquid Effluent Treatment Plants) ofnuclear power stations, in which, depending on the case, the operationsof precipitating the dissolved elements, neutralizing the liquids,evaporating the water and drying of the precipitates are carried out.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows one particular device for milling a conducting carbonmaterial.

FIG. 2 illustrates the particle size curves for the graphite powdersobtained in two trials carried out by applying, for each, a differentnumber of pulses.

DESCRIPTION OF ONE PARTICULAR EMBODIMENT OF THE INVENTION

EXAMPLE

FIG. 1 illustrates a particular device used within the context of thisexample.

This device comprises a sealed reactor 1 made of non-conductingmaterial, for example polyethylene. The bottom of the reactor is aconducting plate, constituting the earth electrode 2 connected to ahigh-voltage generator 3, of the Marx generator type. This generatorsupplies a high-voltage electrode 4, the distance of which from theearth electrode 2 can be adjusted. A block of nuclear graphite 5 restson the bottom of the reactor, the said block being completely immersedin a liquid medium 6. High-voltage pulses are directed substantiallytowards the block 5, thus releasing fragments 7 of the said initialblock 5. The high-voltage pulses are produced in the form of electricarcs between the high-voltage electrode and the earth electrode, thepotential difference applied between these two electrodes depending onthe distance between these two electrodes.

A vent 8 for the gases possibly produced during the milling is providedso as to avoid any overpressure phenomenon.

A block of nuclear graphite, with a mass of around 60 g, is placed inthe reactor described above. A Marx generator used delivers pulses ofthe order of 1 kJ at a frequency of 10 Hz and with a duration of 1 μs.

The block of nuclear graphite is covered with water so as to becompletely immersed.

Two series of trials were carried out:

-   -   a first series in which the set number of pulses was 720; and    -   a second series in which the set number of pulses was about        5000.

The results of these trials are plotted in FIG. 2, which shows theparticle size distribution of the graphite powder obtained. The size Ø(in μm) of the graphite particles obtained is plotted on the x-axis ofthe graph, the scale being logarithmic, and the percentage % of thenumber of particles having a given size relative to the total number ofparticles is plotted on the y-axis of the graph. The sizes of thegraphite particles obtained are determined using the Coulter methodbased on the principle of laser scattering. In this example, thesampling was taken only at the top of the reactor, without stirring theassembly.

Curve (a) shows the size distribution of the particles formed for 720pulses, while curve (b) shows the distribution for about 5000 pulses.

As regards the trial with 720 pulses, a mean particle size of around 800μm is obtained. As regards the trial with about 5000 pulses, a meanparticle size of around 100 μm is obtained. These two trials clearlyshow that the effectiveness of the milling increases with the number ofpulses.

Once the energy, frequency and duration of the pulses have been set, aperson skilled in the art will, by experiment, set a suitable number ofpulses depending on the desired particle size distribution.

1-5. (Canceled).
 6. A process for the milling of a nuclear graphitecontaminated with radioelements, the process comprising: subjecting thegraphite, immersed in a liquid medium, to high-voltage pulses, theliquid medium having a resistivity such that, owing to the effect of theenergy conveyed by the pulses, electric arcs are initiated and, uponcontact with the graphite, break the carbon-carbon bonds that make upthe graphite, the number of high-voltage pulses being set so as toobtain graphite particles of a given particle size.
 7. The millingprocess according to claim 6, wherein the energy of the high-voltagepulses is from 10 J to 100 kJ.
 8. The milling process according to claim6, wherein the high-voltage pulses have a duration ranging from 200 nsto 100 μs.
 9. The milling process according to claim 6, wherein thehigh-voltage pulses have a frequency ranging from 1 Hz to 1000 Hz. 10.The milling process according to claim 6, wherein the liquid medium iswater.